A large number of different evaluation methods exists in the field of optical metrology. Many of these employ interferometry with incoherent light or short coherence light in the visible and near infrared range. Because of the spectrum of light, the resolution of interferometric measurement methods is better than 1 μm in principle. As a result, these methods are particularly suitable for highly precise measurement of lengths, for example, in vibration analysis or surface inspection of workpieces. Specifically, interferometry with short coherence light or white light is an established technique, for example, for measuring roughness or for measuring microstructures on precision manufactured parts.
A majority of the measuring systems based on this technique, however, involve large, relatively inflexible devices which are intended exclusively for carrying out a measurement task. In order to measure small voids as well, it is necessary to use small or even miniaturized measurement probes. In this case, optical microprobes of this kind make up a component part of an optical sensor system and can be applied to the object surface, where they detect changes in distance between the microprobe and the object surface with great precision.
Because short coherence light is used, the requirements for optical components in such systems are very high. So as not to lose the ability of the light to interfere, no dispersion differences may occur between measurement light bundles and reference light bundles. Moreover, in order to achieve a high spatial resolution and, at the same time, insensitivity to locally oblique surfaces in the object surface to be measured, it is advantageous when the measurement light bundle exits the measurement probe with the highest possible numerical aperture and therefore impinges on the object surface in a sharply focused manner.
Optical microprobe technology is a field whose potential has barely been tapped in industrial manufacturing. The reason for this consists primarily in the lack of availability of suitable, inexpensive sensor systems which are able to meet requirements with respect to miniaturization, accuracy, robustness and measuring speed.
A simple and flexible variant is described in the publication APPLIED OPTICS, Vol. 46, No. 17. The description relates to the combination of a Michelson interferometer used as receiver and a fiber-coupled microprobe which is itself made up of fiber-optic components and acts as a Fizeau interferometer. The Fizeau interferometer causes the light which is used for interferometry to be split into a measurement light bundle and a reference light bundle. A portion of a gradient index fiber with a maximum numerical aperture of NA =0.11 is arranged at the output of the microprobe for beam-shaping the measurement light bundle on the object surface. The desired focusing is adjusted in the microprobe by means of the length of the gradient index fiber portion. An improved signal quality is achieved here through the use of two light sources emitting in different spectral ranges allowing a more precise evaluation of the interferogram. However, a disadvantage in this solution is the virtually collimated parallel measurement light bundle which renders the microprobe undesirably sensitive to inclinations and irregularities on the object surface because the measurement signal fed back into the light-conducting fiber is severely weakened by a possible incline on the object surface. However, the in-probe reference light bundle of the Fizeau probe which is generated at the light output surface of the gradient index fiber portion is not subject to any dispersion effects with respect to the actual measurement signal.
German Patent 10 2007 039 556 B3 likewise describes a fiber-optic microprobe in which a distance from the object surface can be kept as short as possible at higher numerical apertures (NA≧0.1) by means of optical components which are individually adapted to the measurement task. To prevent the beam traveling freely between the microprobe and object surface from undergoing any further refraction and, therefore, a further increase in the chromatic aberration, the end of the microprobe is additionally constructed as a concave surface whose focus lies in the light-conducting fiber core and in the measurement spot. The concave surface serves to generate the reference light bundle. Since there is no dispersive medium between the measurement spot and the concave surface, no wavelength-dependent path differences result in this case between the measurement light bundle and reference light bundle. However, the individually adapted optical elements and the very small concave end which is exactly adapted to the focal length of the microprobe require a high expenditure for production of the microprobe. Another drawback of concave reference surfaces near the actual measurement spot, aside from the reduced divergence of the measurement light bundle, is that the concave surface must be positioned with micron to submicron accuracy with respect to the light emergence point of the single-mode (monomode) fiber of the microprobe because the reference light bundle reflected at the concave surface can only be efficiently fed back into the light-conducting fiber when there is a very good overlap of the virtual focus of the concave surface with the light-conducting fiber core, whose size is typically between 2 μm and 5 μm. This requirement significantly complicates the mounting and alignment of all of the components relative to one another and assumes a long-term mechanical and climatic stability of the overall system that is difficult to achieve. Therefore, it is hardly possible to miniaturize the microprobe while retaining robustness at the same time.